1. Field of the invention
The present invention relates in general to non-destructive ultrasonic testing, and in particular to a method and system for determining material properties of an object using ultrasonic attenuation.
2. Description of prior art
Ultrasonic attenuation is a measure of the decay of ultrasonic strength during propagation through a material, and can be used to assess properties of materials. For example, the ultrasonic attenuation is directly related to grain size in a polycrystalline solid, e.g. in most metals. The grain size has a strong impact on important mechanical properties of polycrystalline solids. Ultrasonic attenuation can also be used to determine the concentration and size of particles included in a medium, in either a solid or liquid state, or for determining a porosity distribution in a composite material. Another example is the combined use of ultrasonic attenuation and velocity to characterize relaxation behavior and viscoelastic properties of polymeric materials. The physical mechanisms that produce ultrasonic attenuation include scattering and absorption, both of which can be used to characterize material properties. These physical mechanisms (scattering and absorption) are frequency dependent, which is to say that at different frequencies, different attenuation rates are observed. It is known to perform ultrasonic attenuation measurements using a narrowband system (in which case measurement is typically reported at a center frequency), or using a broadband system involving a frequency domain analysis.
One common technique used for attenuation measurement is known as pulse-echo configuration (reflection mode), with ultrasound generated and detected by a piezoelectric transducer coupled to the test object by a couplant or a solid or liquid buffer (i.e. a coupling medium). Another technique is the through-transmission configuration (or transmission mode), where two transducers that face each other on opposite sides of the test object are used for sending and receiving the ultrasound. The through-transmission configuration requires access to both sides of the material. Also according to the through-transmission configuration, the transducer pair must be perfectly matched or fully characterized and preferably aligned with the test object using a coupling medium on both sides. A third technique (known as pitch-catch configuration) involves a pair of transducers separated from each other by a distance, usually on the same side of the test object. The pitch-catch configuration has been used for measuring ultrasonic attenuation of Rayleigh surface waves, Lamb waves, as well as ultrasonic attenuation of bulk waves.
These configurations for attenuation measurement can also be used with non-contact ultrasonic generation and detection techniques, using electromagnetic acoustic transducers (EMATs), air coupled transducers, or laser-ultrasonics. Laser-ultrasonics use one laser with a short pulse for generation of ultrasonic waves. The transfer of energy from the laser to the ultrasonic waves can occur in the thermoelastic regime, where thermal expansion on a surface due to the sudden laser heating is responsible for generating an ultrasonic pulse, or in an ablation regime wherein the laser energy removes a thin layer of the surface, and produces a plasma which induces the ultrasonic waves.
A second laser with longer pulse (or even a continuous wave) is typically used for detection. The second laser illuminates a detection location on the surface of the test object and a phase or frequency shift in the reflected light due to the arrival of an attenuated ultrasonic pulse at the detection location is measured using an optical interferometric system. Interferometric systems for ultrasonic detection known in the art include those based on time-delay interferometry, and systems based on nonlinear optics for wavefront adaptation, as explained by Monchalin J.-P., in “Laser-ultrasonics: from the laboratory to industry”, Review of Progress in Quantitative Nondestructive evaluation Vol. 23A, ed. by D. O. Thompson and D. E. Chimenti, AIP Conf. Proc., New York, 2004, pp. 3-31. Generation and detection of ultrasound are performed at a distance and eliminate the need for coupling liquid and the alignment requirements of conventional ultrasonics.
Using any of the above configurations, conventional methods of measuring ultrasonic attenuation involve determining a decay of a detected ultrasonic pulse (amplitude) for two propagation distances in the material, e.g. using two echo signals that reverberate between faces of the test object. The attenuation is calculated by comparing amplitudes of the two echoes at each frequency, as explained by A. Vary in Nondestructive Testing Handbook, V. 7, 2nd Edition, pp. 383-431 ASNT (1991).
Unfortunately the calculated attenuation is affected by noise of both echoes i.e. uncertainties of both measurements reduce the accuracy of the attenuation value. When the test object is thick and/or made of high attenuation material, the second echo has a poor signal-to-noise ratio (SNR). In such cases, the two-echo attenuation method may not permit accurate measurement, and the SNR of the first echo is not fully exploited. Given the limitations of the conventional two-echo method, the use of a single echo approach to determine ultrasonic attenuation is strongly desirable. However, the amplitude of an echo is also dependent on the generation strength, coupling efficiency, detection efficiency, etc. In the conventional two-echo method, the comparison with an echo inherently accounts for all of these factors, providing a normalized reading.
Another difficulty with the conventional two-echo method is the need for correction of diffraction effects of the ultrasonic pulse to obtain the intrinsic ultrasonic attenuation that is attributable to the test object. While simplified theoretical models have been used to calculate a correction for diffraction for simple geometries of the test object, the diffraction behavior in real situations can be more complex.
The elimination of variations caused by the generation strength, coupling efficiency, detection efficiency, and diffraction to produce a fully normalized intrinsic ultrasonic attenuation spectrum is the most challenging task for the use of a single echo to determine material properties. For some embodiments of the pulse-echo configuration, normalization may be performed using the ultrasonic pulse that is reflected by the surface of the test object (i.e. an entrance echo) to characterize the strength of the generated ultrasonic pulse, permitting the amplitude of the single echo interaction signal to be used. For the through-transmission configuration, normalization is often made by comparison with the pulse propagating through the coupling medium in the absence of the test object. This configuration requires access to both sides of the material, which may not be possible, or preferred, in some industrial applications. Furthermore, the use of an entrance echo, or sample removed reading in the above cases does not remove the need for diffraction correction to obtain the intrinsic material attenuation. A model is still required with the precise knowledge of the characteristics of the system used.
The use of a single echo in a laser-ultrasonic technique to measure a material property in comparison with attenuation of a reference material is taught in the U.S. Pat. No. 6,684,701 to Dubois et al. Dubois et al. teach a method for ultrasonic measuring of porosity of a sample composite material by accessing only one side of the sample composite material. The method involves measuring a sample ultrasonic signal from the sample composite material, normalizing the sample ultrasonic signal relative to the surface displacement at generation on the sample composite material, and isolating a sample back-wall echo from the sample ultrasonic signal. A sample frequency spectrum of the sample back-wall echo is then determined. Next, the method includes the steps of measuring a reference ultrasonic signal from a reference composite material, normalizing the reference ultrasonic signal relative to the surface displacement at generation on the reference composite material, and isolating a reference back-wall echo from the sample ultrasonic signal. A reference frequency spectrum of the reference back-wall echo is then determined. The invention further includes deriving the ultrasonic attenuation of the sample composite material as the ratio of the sample frequency spectrum to the reference frequency spectrum over a predetermined frequency range. Comparing the derived ultrasonic attenuation to predetermined attenuation standards permits the evaluation the porosity of the sampled composite material.
The method of Dubois et al. is limited to the pulse-echo configuration described above. According to Dubois et al., it is necessary to compensate for variance in the generation strength and detection efficiency, by comparing the amplitude with a surface displacement at generation on the surface of the test object to normalize each echo. This approach can only be applied to laser-generated ultrasound in the thermoelastic regime. Further the method according to Dubois et al. does not account for variations caused by penetration of light through the surface of the test object. Unfortunately the surface displacement is not an accurate measure of energy of the ultrasonic pulse, as it does not account for contributions from penetrating light. For example, in carbon-epoxy composites, the light penetration of the ultrasonic generating laser is dependent upon the thickness of a superficial epoxy layer, which in practice, varies considerably and is difficult to determine. Also, normalization using surface displacement is not applicable for laser-generated ultrasound in the ablation regime where the generation laser produces a plasma. While a strong signal may be detected at generation (particularly when the pulse echo method is used), the strong signal, while including a contribution of the surface displacement, is primarily caused by a refractive index perturbation of plasma, which is highly variable, and cannot be relied upon to gage the strength of the ultrasonic pulse.
There therefore remains a need for a method and system for deriving an ultrasonic attenuation measurement using a single echo that compensates for diffraction.